1. Field of the Invention
The invention relates to distribution of digital content.
2. Related Art
Distribution of digital content for media streams, such as for example movies, is subject to several problems. One problem is that it is easy to make exact copies of digital content, thus allowing any recipient of that content to redistribute it, whether authorized or not. It would be advantageous to be able to distribute digital content, particularly digital content for media streams, without fear of its unauthorized distribution. This would be particularly advantageous when it is desired to distribute digital content using a communication link, such as for example a computer network or other technique for distribution to end viewers (for example, either on demand, in anticipation of future demand, or in response to something else).
One known solution is to mark digital content with a “fingerprint,” that is, information sufficient to identify the recipient of the digital content, so that an unauthorized distributor of the content can be determined, thus hopefully deterring potential unauthorized distributors. Other known solutions include marking digital content with a “watermark,” that is, information sufficient to identify the digital content itself. Thus for example, if a recipient of the digital content distributes it without authorization, that recipient can be identified and called to account for that activity. However, fingerprinting and watermarking of digital content are subject to several problems. Among these problems are several possible attacks, notably including the possibility of reformatting the media streams into new digital content representing the media streams, with the effect that the new digital content is significantly different from the original digital content, and with the effect that detection of identifying information from the original digital content is made substantially more difficult. Differences can include new data formats, new frame rates, new pixel resolution, new viewing angles, and the like. Such differences, and any other attack with the effect that new presentation of the media streams in which new digital content (such as its data pixels) cannot be easily aligned with the original data content (such as original data pixels) are herein sometimes called “de-synchronization attacks.”
Fingerprinting and watermarking techniques often depend on being able to compare elements of the original digital content, such as for example data found in individual pixels, with related elements of the digital content after an attack, such as for example a de-synchronization attack. For a first example, the Stirmark benchmarking tool induces local bending of the media stream, in the expectation that is difficult to undo that local bending, so as to closely associate the altered digital content with the original digital content. For a second example, digital to analog conversion of the original digital content, followed by analog to digital conversion back (and possibly combined with some form of analog distortion), thus producing new digital content, is likely to disassociate those individual pixels that are grouped together into blocks. This would have the probable effect that individual pixels after both conversions are performed have new positions that do not coincide with their positions before the attack, and would have the possible effect that individual pixels from an original single block before the attack would lie in several different blocks after the attack.
It would be advantageous to provide a technique by which desynchronization attacks can be thwarted, with the effect that recipients responsible for distributed digital content can be identified notwithstanding such attacks.